Germanium (Ge) has many characteristics which render it a highly desirable material for integration into semiconductor structures. Some of the useful properties of Ge are that it has a smaller bandgap than silicon (Si), yet is has about three orders of magnitude higher intrinsic carrier concentration and therefore a higher conductivity. It has a low dopant activation temperature and doesn't require special processing for activation. It can be used as a dopant source and it is optically active. For these reasons, Ge has many potential uses, such as a replacement material in many applications where polycrystalline silicon has previously been utilized. A potential disadvantage of Ge, however, is that it exhibits intrinsically selective growth over Si. In other words, Ge will grow on Si but will not grow on a number of other mediums, such as oxides.
A number of techniques have emerged for depositing Ge on substrates, including chemical vapor deposition (CVD), laser induced chemical vapor deposition (LICVD) and ultra high vacuum chemical vapor deposition (UHV/CVD).
Examples of prior deposition methods for growing Ge utilizing CVD are disclosed in the following publications.
Maenpaa, et al., "The heteroepitaxy of Ge on Si: A comparison of chemical vapor and vacuum deposited layers", J. Appl. Phys. 53(2), (February 1982), 1076-1083, (Maenpaa et al.) describes a CVD experiment performed utilizing GeH.sub.4 as a carrier gas with a pressure of 2-13 Torr and maintaining the Si temperature between 500.degree.-900.degree. C.
Hitoshi Ishii, "Manufacture of Semiconductor Device and Equipment Therefor", Japanese Patent publication 62-179113, application no. 61-20283, (Ishii) discloses the deposition of Ge using GeH.sub.4 as a carrier gas in a CVD chamber at 450.degree. C.
U.S. Pat. No. 3,473,978 (Jackson), entitled "Epitaxial Growth of Germanium", teaches a method for the nucleation and growth of monocrystalline germanium on a Si substrate which comprises epitaxially growing a layer of monocrystalline Si at a temperature of at least 900.degree. C., then cooling the Si below 670.degree. C., followed by the nucleation and growth of Ge.
An example of a LICVD technique for depositing Ge is disclosed in U.S. Pat. No. 4,681,640 (Stanley) wherein polycrystalline Germanium films are formed by irradiating germane related compounds with CO.sub.2 laser energy at pressures above 3.5 Torr.
Examples of UHV/CVD techniques for depositing Ge.sub.x Si.sub.1-x alloys are disclosed in the following references.
Meyerson, Uram and LeGoues, "Cooperative growth phenomena in silicon/germanium low-temperature epitaxy", (Appl. Phys. Lett. 53 (25), 19 Dec. 1988, 1988 American Institute of Physics), (MEYERSON ET AL.), which teaches deposition of alloys of composition 0.ltoreq.Ge/Si.ltoreq.0.20 using UHV/CVD and a temperature of 550.degree. C.
Racanelli and Greve, "Growth of Epitaxial Layers of Ge.sub.x Si.sub.1-x by UHV/CVD", (Mat. Res. Soc. Symp. Proc. Vol. 198, 1990 Materials Research Society), (RACANELLI et al.), which teaches a method of growing epitaxial layers of a Ge.sub.x Si.sub.1-x composition on Si at temperatures between 577.degree. and 665.degree. C.
None of the aforementioned CVD or UHV/CVD references, however, describes a practical method for growing Ge on monocrystalline Si in a manufacturing environment. In addition, all of the prior methods teach growing Ge over only Si related semiconductor mediums. A practical method for growing Ge nonselectively over other materials, such as oxides in a high throughput manufacturing environment is highly desirable.